Re: [PATCH] Reducing number of alias checks in vectorization.

[Cong Hou <congh at google dot com>]

On Wed, Oct 2, 2013 at 2:18 PM, Xinliang David Li <davidxl@google.com> wrote:
> On Wed, Oct 2, 2013 at 4:24 AM, Richard Biener <rguenther@suse.de> wrote:
>> On Tue, 1 Oct 2013, Cong Hou wrote:
>>
>>> When alias exists between data refs in a loop, to vectorize it GCC
>>> does loop versioning and adds runtime alias checks. Basically for each
>>> pair of data refs with possible data dependence, there will be two
>>> comparisons generated to make sure there is no aliasing between them
>>> in each iteration of the vectorized loop. If there are many such data
>>> refs pairs, the number of comparisons can be very large, which is a
>>> big overhead.
>>>
>>> However, in some cases it is possible to reduce the number of those
>>> comparisons. For example, for the following loop, we can detect that
>>> b[0] and b[1] are two consecutive member accesses so that we can
>>> combine the alias check between a[0:100]&b[0] and a[0:100]&b[1] into
>>> checking a[0:100]&b[0:2]:
>>>
>>> void foo(int*a, int* b)
>>> {
>>>    for (int i = 0; i < 100; ++i)
>>>     a[i] = b[0] + b[1];
>>> }
>>>
>>> Actually, the requirement of consecutive memory accesses is too
>>> strict. For the following loop, we can still combine the alias checks
>>> between a[0:100]&b[0] and a[0:100]&b[100]:
>>>
>>> void foo(int*a, int* b)
>>> {
>>>    for (int i = 0; i < 100; ++i)
>>>     a[i] = b[0] + b[100];
>>> }
>>>
>>> This is because if b[0] is not in a[0:100] and b[100] is not in
>>> a[0:100] then a[0:100] cannot be between b[0] and b[100]. We only need
>>> to check a[0:100] and b[0:101] don't overlap.
>>>
>>> More generally, consider two pairs of data refs (a, b1) and (a, b2).
>>> Suppose addr_b1 and addr_b2 are basic addresses of data ref b1 and b2;
>>> offset_b1 and offset_b2 (offset_b1 < offset_b2) are offsets of b1 and
>>> b2, and segment_length_a, segment_length_b1, and segment_length_b2 are
>>> segment length of a, b1, and b2. Then we can combine the two
>>> comparisons into one if the following condition is satisfied:
>>>
>>> offset_b2- offset_b1 - segment_length_b1 < segment_length_a
>>>
>>>
>>> This patch detects those combination opportunities to reduce the
>>> number of alias checks. It is tested on an x86-64 machine.
>>
>> Apart from the other comments you got (to which I agree) the patch
>> seems to do two things, namely also:
>>
>> +  /* Extract load and store statements on pointers with zero-stride
>> +     accesses.  */
>> +  if (LOOP_REQUIRES_VERSIONING_FOR_ALIAS (loop_vinfo))
>> +    {
>>
>> which I'd rather see in a separate patch (and done also when
>> the loop doesn't require versioning for alias).
>
> yes.
>
>>
>> Also combining the alias checks in vect_create_cond_for_alias_checks
>> is nice but doesn't properly fix the use of the
>> vect-max-version-for-alias-checks param
>
> Yes. The handling of this should be moved to
> 'vect_prune_runtime_alias_test_list' to avoid premature decisions.
>
>
>
>>which currently inhibits
>> vectorization of the HIMENO benchmark by default (and make us look bad
>> compared to LLVM).
>
> Here is a small reproducible:
>
> struct  A {
>   int *base;
>   int offset;
>   int offset2;
>   int offset3;
>   int offset4;
>   int offset5;
>   int offset6;
>   int offset7;
>   int offset8;
> };
>
> void foo (struct A * ar1, struct A* ar2)
> {
>       int i;
>       for (i = 0; i < 10000; i++)
>         {
>            ar1->base[i]  = 2*ar2->base[i] + ar2->offset + ar2->offset2
> + ar2->offset3 + ar2->offset4 + ar2->offset5 + ar2->offset6; /* +
> ar2->offset7 + ar2->offset8;*/
>         }
> }
>
> GCC trunk won't vectorize it at O2 due to the limit.
>
>
> There is another problem we should be tracking: GCC no longer
> vectorize the loop (with large
> --param=vect-max-version-for-alias-checks=40) when -fno-strict-alias
> is specified.   However with additional runtime alias check, the loop
> should be vectorizable.


The problem can be reproduced by the following loop:


void foo (int* a, int** b)
{
  int i;
  for (i = 0; i < 1000; ++i)
    a[i] = (*b)[i];
}


When -fno-strict-aliasing is specified, the basic address of (*b)[i]
which is *b could be modified by a[i] if alias exists between them.
This forbids GCC from making the basic address of (*b)[i] a loop
invariant, and hence could not do vectorization. Although we can still
check the aliasing between a[i] and *b from a and b (b < a or b >=
a+1000), it must be done before vectorization and after this we have
to do one more nested aliasing check (a and *b now):


void foo (int* a, int** b)
{
  int i;
  if ((int*)b < a || (int*)b >= a+1000)
  {
    int* c = *b;
    for (i = 0; i < 1000; ++i)
      a[i] = c[i];
  }
  else
  {
    for (i = 0; i < 1000; ++i)
      a[i] = (*b)[i];
  }
}

I think this is too complicated to be able to get any benefit for us.


thanks,
Cong



>
> David
>
>
>>
>> So I believe this merging should be done incrementally when
>> we collect the DDRs we need to test in vect_mark_for_runtime_alias_test.
>>
>> Thanks for working on this,
>> Richard.
>>
>>>
>>> thanks,
>>> Cong
>>>
>>>
>>>
>>> Index: gcc/tree-vect-loop-manip.c
>>> ===================================================================
>>> --- gcc/tree-vect-loop-manip.c (revision 202662)
>>> +++ gcc/tree-vect-loop-manip.c (working copy)
>>> @@ -19,6 +19,10 @@ You should have received a copy of the G
>>>  along with GCC; see the file COPYING3.  If not see
>>>  <http://www.gnu.org/licenses/>.  */
>>>
>>> +#include <vector>
>>> +#include <utility>
>>> +#include <algorithm>
>>> +
>>>  #include "config.h"
>>>  #include "system.h"
>>>  #include "coretypes.h"
>>> @@ -2248,6 +2252,74 @@ vect_vfa_segment_size (struct data_refer
>>>    return segment_length;
>>>  }
>>>
>>> +namespace
>>> +{
>>> +
>>> +/* struct dr_addr_with_seg_len
>>> +
>>> +   A struct storing information of a data reference, including the data
>>> +   ref itself, its basic address, the access offset and the segment length
>>> +   for aliasing checks.  */
>>> +
>>> +struct dr_addr_with_seg_len
>>> +{
>>> +  dr_addr_with_seg_len (data_reference* d, tree addr, tree off, tree len)
>>> +    : dr (d), basic_addr (addr), offset (off), seg_len (len) {}
>>> +
>>> +  data_reference* dr;
>>> +  tree basic_addr;
>>> +  tree offset;
>>> +  tree seg_len;
>>> +};
>>> +
>>> +/* Operator == between two dr_addr_with_seg_len objects.
>>> +
>>> +   This equality operator is used to make sure two data refs
>>> +   are the same one so that we will consider to combine the
>>> +   aliasing checks of those two pairs of data dependent data
>>> +   refs.  */
>>> +
>>> +bool operator == (const dr_addr_with_seg_len& d1,
>>> +  const dr_addr_with_seg_len& d2)
>>> +{
>>> +  return operand_equal_p (d1.basic_addr, d2.basic_addr, 0)
>>> + && operand_equal_p (d1.offset, d2.offset, 0)
>>> + && operand_equal_p (d1.seg_len, d2.seg_len, 0);
>>> +}
>>> +
>>> +typedef std::pair <dr_addr_with_seg_len, dr_addr_with_seg_len>
>>> + dr_addr_with_seg_len_pair_t;
>>> +
>>> +
>>> +/* Operator < between two dr_addr_with_seg_len_pair_t objects.
>>> +
>>> +   This operator is used to sort objects of dr_addr_with_seg_len_pair_t
>>> +   so that we can combine aliasing checks during one scan.  */
>>> +
>>> +bool operator < (const dr_addr_with_seg_len_pair_t& p1,
>>> + const dr_addr_with_seg_len_pair_t& p2)
>>> +{
>>> +  const dr_addr_with_seg_len& p11 = p1.first;
>>> +  const dr_addr_with_seg_len& p12 = p1.second;
>>> +  const dr_addr_with_seg_len& p21 = p2.first;
>>> +  const dr_addr_with_seg_len& p22 = p2.second;
>>> +
>>> +  if (p11.basic_addr != p21.basic_addr)
>>> +    return p11.basic_addr < p21.basic_addr;
>>> +  if (p12.basic_addr != p22.basic_addr)
>>> +    return p12.basic_addr < p22.basic_addr;
>>> +  if (TREE_CODE (p11.offset) != INTEGER_CST
>>> +      || TREE_CODE (p21.offset) != INTEGER_CST)
>>> +    return p11.offset < p21.offset;
>>> +  if (int_cst_value (p11.offset) != int_cst_value (p21.offset))
>>> +    return int_cst_value (p11.offset) < int_cst_value (p21.offset);
>>> +  if (TREE_CODE (p12.offset) != INTEGER_CST
>>> +      || TREE_CODE (p22.offset) != INTEGER_CST)
>>> +    return p12.offset < p22.offset;
>>> +  return int_cst_value (p12.offset) < int_cst_value (p22.offset);
>>> +}
>>> +
>>> +}
>>>
>>>  /* Function vect_create_cond_for_alias_checks.
>>>
>>> @@ -2292,20 +2364,51 @@ vect_create_cond_for_alias_checks (loop_
>>>    if (may_alias_ddrs.is_empty ())
>>>      return;
>>>
>>> +
>>> +  /* Basically, for each pair of dependent data refs store_ptr_0
>>> +     and load_ptr_0, we create an expression:
>>> +
>>> +     ((store_ptr_0 + store_segment_length_0) <= load_ptr_0)
>>> +     || (load_ptr_0 + load_segment_length_0) <= store_ptr_0))
>>> +
>>> +     for aliasing checks. However, in some cases we can decrease
>>> +     the number of checks by combining two checks into one. For
>>> +     example, suppose we have another pair of data refs store_ptr_0
>>> +     and load_ptr_1, and if the following condition is satisfied:
>>> +
>>> +     load_ptr_0 < load_ptr_1  &&
>>> +     load_ptr_1 - load_ptr_0 - load_segment_length_0 < store_segment_length_0
>>> +
>>> +     (this condition means, in each iteration of vectorized loop,
>>> +     the accessed memory of store_ptr_0 cannot be between the memory
>>> +     of load_ptr_0 and load_ptr_1.)
>>> +
>>> +     we then can use only the following expression to finish the
>>> +     alising checks between store_ptr_0 & load_ptr_0 and
>>> +     store_ptr_0 & load_ptr_1:
>>> +
>>> +     ((store_ptr_0 + store_segment_length_0) <= load_ptr_0)
>>> +     || (load_ptr_1 + load_segment_length_1 <= store_ptr_0))
>>> +
>>> +     Note that we only consider that load_ptr_0 and load_ptr_1 have the
>>> +     same basic address.  */
>>> +
>>> +  std::vector<dr_addr_with_seg_len_pair_t> ddrs_with_seg_len;
>>> +
>>> +  /* First, we collect all data ref pairs for aliasing checks.  */
>>> +
>>>    FOR_EACH_VEC_ELT (may_alias_ddrs, i, ddr)
>>>      {
>>>        struct data_reference *dr_a, *dr_b;
>>>        gimple dr_group_first_a, dr_group_first_b;
>>> -      tree addr_base_a, addr_base_b;
>>>        tree segment_length_a, segment_length_b;
>>>        gimple stmt_a, stmt_b;
>>> -      tree seg_a_min, seg_a_max, seg_b_min, seg_b_max;
>>>
>>>        dr_a = DDR_A (ddr);
>>>        stmt_a = DR_STMT (DDR_A (ddr));
>>>        dr_group_first_a = GROUP_FIRST_ELEMENT (vinfo_for_stmt (stmt_a));
>>>        if (dr_group_first_a)
>>> -        {
>>> + {
>>>    stmt_a = dr_group_first_a;
>>>    dr_a = STMT_VINFO_DATA_REF (vinfo_for_stmt (stmt_a));
>>>   }
>>> @@ -2314,20 +2417,11 @@ vect_create_cond_for_alias_checks (loop_
>>>        stmt_b = DR_STMT (DDR_B (ddr));
>>>        dr_group_first_b = GROUP_FIRST_ELEMENT (vinfo_for_stmt (stmt_b));
>>>        if (dr_group_first_b)
>>> -        {
>>> + {
>>>    stmt_b = dr_group_first_b;
>>>    dr_b = STMT_VINFO_DATA_REF (vinfo_for_stmt (stmt_b));
>>>   }
>>>
>>> -      addr_base_a
>>> - = fold_build_pointer_plus (DR_BASE_ADDRESS (dr_a),
>>> -   size_binop (PLUS_EXPR, DR_OFFSET (dr_a),
>>> -       DR_INIT (dr_a)));
>>> -      addr_base_b
>>> - = fold_build_pointer_plus (DR_BASE_ADDRESS (dr_b),
>>> -   size_binop (PLUS_EXPR, DR_OFFSET (dr_b),
>>> -       DR_INIT (dr_b)));
>>> -
>>>        if (!operand_equal_p (DR_STEP (dr_a), DR_STEP (dr_b), 0))
>>>   length_factor = scalar_loop_iters;
>>>        else
>>> @@ -2335,24 +2429,149 @@ vect_create_cond_for_alias_checks (loop_
>>>        segment_length_a = vect_vfa_segment_size (dr_a, length_factor);
>>>        segment_length_b = vect_vfa_segment_size (dr_b, length_factor);
>>>
>>> +      dr_addr_with_seg_len_pair_t dr_with_seg_len_pair
>>> +  (dr_addr_with_seg_len
>>> +       (dr_a, DR_BASE_ADDRESS (dr_a),
>>> + size_binop (PLUS_EXPR, DR_OFFSET (dr_a), DR_INIT (dr_a)),
>>> + segment_length_a),
>>> +   dr_addr_with_seg_len
>>> +       (dr_b, DR_BASE_ADDRESS (dr_b),
>>> + size_binop (PLUS_EXPR, DR_OFFSET (dr_b), DR_INIT (dr_b)),
>>> + segment_length_b));
>>> +
>>> +      if (dr_with_seg_len_pair.first.basic_addr >
>>> +  dr_with_seg_len_pair.second.basic_addr)
>>> + std::swap (dr_with_seg_len_pair.first, dr_with_seg_len_pair.second);
>>> +
>>> +      ddrs_with_seg_len.push_back (dr_with_seg_len_pair);
>>> +    }
>>> +
>>> +  /* Second, we sort the collected data ref pairs so that we can scan
>>> +     them once to combine all possible aliasing checks.  */
>>> +
>>> +  std::sort (ddrs_with_seg_len.begin(), ddrs_with_seg_len.end());
>>> +
>>> +  /* Remove duplicate data ref pairs.  */
>>> +  ddrs_with_seg_len.erase (std::unique (ddrs_with_seg_len.begin(),
>>> + ddrs_with_seg_len.end()),
>>> +   ddrs_with_seg_len.end());
>>> +
>>> +  /* We then scan the sorted dr pairs and check if we can combine
>>> +     alias checks of two neighbouring dr pairs.  */
>>> +
>>> +  for (size_t i = 1; i < ddrs_with_seg_len.size (); ++i)
>>> +    {
>>> +      dr_addr_with_seg_len& dr_a1 = ddrs_with_seg_len[i-1].first;
>>> +      dr_addr_with_seg_len& dr_b1 = ddrs_with_seg_len[i-1].second;
>>> +      dr_addr_with_seg_len& dr_a2 = ddrs_with_seg_len[i].first;
>>> +      dr_addr_with_seg_len& dr_b2 = ddrs_with_seg_len[i].second;
>>> +
>>> +      if (dr_a1 == dr_a2)
>>> + {
>>> +  if (dr_b1.basic_addr != dr_b2.basic_addr
>>> +      || TREE_CODE (dr_b1.offset) != INTEGER_CST
>>> +      || TREE_CODE (dr_b2.offset) != INTEGER_CST)
>>> +    continue;
>>> +
>>> +  int diff = int_cst_value (dr_b2.offset) -
>>> +     int_cst_value (dr_b1.offset);
>>> +
>>> +  gcc_assert (diff > 0);
>>> +
>>> +  if (diff <= vect_factor
>>> +      || (TREE_CODE (dr_b1.seg_len) == INTEGER_CST
>>> +  && diff - int_cst_value (dr_b1.seg_len) < vect_factor)
>>> +      || (TREE_CODE (dr_b1.seg_len) == INTEGER_CST
>>> +  && TREE_CODE (dr_a1.seg_len) == INTEGER_CST
>>> +  && diff - int_cst_value (dr_b1.seg_len) <
>>> +     int_cst_value (dr_a1.seg_len)))
>>> +    {
>>> +      if (dump_enabled_p ())
>>> + {
>>> +  dump_printf_loc
>>> +      (MSG_NOTE, vect_location,
>>> +       "combining two runtime checks for data references ");
>>> +  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b1.dr));
>>> +  dump_printf (MSG_NOTE, " and ");
>>> +  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b2.dr));
>>> +  dump_printf (MSG_NOTE, "\n");
>>> + }
>>> +
>>> +      dr_b1.seg_len = size_binop (PLUS_EXPR,
>>> +  dr_b2.seg_len, size_int (diff));
>>> +      ddrs_with_seg_len.erase (ddrs_with_seg_len.begin () + i);
>>> +      --i;
>>> +    }
>>> + }
>>> +      else if (dr_b1 == dr_b2)
>>> + {
>>> +  if (dr_a1.basic_addr != dr_a2.basic_addr
>>> +      || TREE_CODE (dr_a1.offset) != INTEGER_CST
>>> +      || TREE_CODE (dr_a2.offset) != INTEGER_CST)
>>> +    continue;
>>> +
>>> +  int diff = int_cst_value (dr_a2.offset) -
>>> +     int_cst_value (dr_a1.offset);
>>> +
>>> +  gcc_assert (diff > 0);
>>> +
>>> +  if (diff <= vect_factor
>>> +      || (TREE_CODE (dr_a1.seg_len) == INTEGER_CST
>>> +  && diff - int_cst_value (dr_a1.seg_len) < vect_factor)
>>> +      || (TREE_CODE (dr_a1.seg_len) == INTEGER_CST
>>> +  && TREE_CODE (dr_b1.seg_len) == INTEGER_CST
>>> +  && diff - int_cst_value (dr_a1.seg_len) <
>>> +     int_cst_value (dr_b1.seg_len)))
>>> +    {
>>> +      if (dump_enabled_p ())
>>> + {
>>> +  dump_printf_loc
>>> +      (MSG_NOTE, vect_location,
>>> +       "combining two runtime checks for data references ");
>>> +  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a1.dr));
>>> +  dump_printf (MSG_NOTE, " and ");
>>> +  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a2.dr));
>>> +  dump_printf (MSG_NOTE, "\n");
>>> + }
>>> +
>>> +      dr_a1.seg_len = size_binop (PLUS_EXPR,
>>> +  dr_a2.seg_len, size_int (diff));
>>> +      ddrs_with_seg_len.erase (ddrs_with_seg_len.begin () + i);
>>> +      --i;
>>> +    }
>>> + }
>>> +    }
>>> +
>>> +  for (size_t i = 0, s = ddrs_with_seg_len.size (); i < s; ++i)
>>> +    {
>>> +      const dr_addr_with_seg_len& dr_a = ddrs_with_seg_len[i].first;
>>> +      const dr_addr_with_seg_len& dr_b = ddrs_with_seg_len[i].second;
>>> +      tree segment_length_a = dr_a.seg_len;
>>> +      tree segment_length_b = dr_b.seg_len;
>>> +
>>> +      tree addr_base_a
>>> + = fold_build_pointer_plus (dr_a.basic_addr, dr_a.offset);
>>> +      tree addr_base_b
>>> + = fold_build_pointer_plus (dr_b.basic_addr, dr_b.offset);
>>> +
>>>        if (dump_enabled_p ())
>>>   {
>>>    dump_printf_loc (MSG_NOTE, vect_location,
>>> -                           "create runtime check for data references ");
>>> -  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a));
>>> +   "create runtime check for data references ");
>>> +  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a.dr));
>>>    dump_printf (MSG_NOTE, " and ");
>>> -  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b));
>>> -          dump_printf (MSG_NOTE, "\n");
>>> +  dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b.dr));
>>> +  dump_printf (MSG_NOTE, "\n");
>>>   }
>>>
>>> -      seg_a_min = addr_base_a;
>>> -      seg_a_max = fold_build_pointer_plus (addr_base_a, segment_length_a);
>>> -      if (tree_int_cst_compare (DR_STEP (dr_a), size_zero_node) < 0)
>>> +      tree seg_a_min = addr_base_a;
>>> +      tree seg_a_max = fold_build_pointer_plus (addr_base_a, segment_length_a);
>>> +      if (tree_int_cst_compare (DR_STEP (dr_a.dr), size_zero_node) < 0)
>>>   seg_a_min = seg_a_max, seg_a_max = addr_base_a;
>>>
>>> -      seg_b_min = addr_base_b;
>>> -      seg_b_max = fold_build_pointer_plus (addr_base_b, segment_length_b);
>>> -      if (tree_int_cst_compare (DR_STEP (dr_b), size_zero_node) < 0)
>>> +      tree seg_b_min = addr_base_b;
>>> +      tree seg_b_max = fold_build_pointer_plus (addr_base_b, segment_length_b);
>>> +      if (tree_int_cst_compare (DR_STEP (dr_b.dr), size_zero_node) < 0)
>>>   seg_b_min = seg_b_max, seg_b_max = addr_base_b;
>>>
>>>        part_cond_expr =
>>> @@ -2477,6 +2696,81 @@ vect_loop_versioning (loop_vec_info loop
>>>        adjust_phi_and_debug_stmts (orig_phi, e, PHI_RESULT (new_phi));
>>>      }
>>>
>>> +  /* Extract load and store statements on pointers with zero-stride
>>> +     accesses.  */
>>> +  if (LOOP_REQUIRES_VERSIONING_FOR_ALIAS (loop_vinfo))
>>> +    {
>>> +
>>> +      /* In the loop body, we iterate each statement to check if it is a load
>>> + or store. Then we check the DR_STEP of the data reference.  If
>>> + DR_STEP is zero, then we will hoist the load statement to the loop
>>> + preheader, and move the store statement to the loop exit.  */
>>> +
>>> +      for (gimple_stmt_iterator si = gsi_start_bb (loop->header);
>>> +   !gsi_end_p (si); )
>>> + {
>>> +  gimple stmt = gsi_stmt (si);
>>> +  stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
>>> +  struct data_reference *dr = STMT_VINFO_DATA_REF (stmt_info);
>>> +
>>> +
>>> +  if (dr && integer_zerop (DR_STEP (dr)))
>>> +    {
>>> +      if (DR_IS_READ (dr))
>>> + {
>>> +  if (dump_file)
>>> +    {
>>> +      fprintf (dump_file,
>>> +       "Hoist the load to outside of the loop:\n");
>>> +      print_gimple_stmt (dump_file, stmt, 0,
>>> + TDF_VOPS|TDF_MEMSYMS);
>>> +    }
>>> +
>>> +  basic_block preheader = loop_preheader_edge (loop)->src;
>>> +  gimple_stmt_iterator si_dst = gsi_last_bb (preheader);
>>> +  gsi_move_after (&si, &si_dst);
>>> + }
>>> +      else
>>> + {
>>> +  gimple_stmt_iterator si_dst =
>>> +      gsi_last_bb (single_exit (loop)->dest);
>>> +  gsi_move_after (&si, &si_dst);
>>> + }
>>> +              continue;
>>> +    }
>>> +  else if (!dr)
>>> +          {
>>> +            bool hoist = true;
>>> +            for (size_t i = 0; i < gimple_num_ops (stmt); i++)
>>> +            {
>>> +              tree op = gimple_op (stmt, i);
>>> +              if (TREE_CODE (op) == INTEGER_CST
>>> +                  || TREE_CODE (op) == REAL_CST)
>>> +                continue;
>>> +              if (TREE_CODE (op) == SSA_NAME)
>>> +              {
>>> +                gimple def = SSA_NAME_DEF_STMT (op);
>>> +                if (def == stmt
>>> +                    || gimple_nop_p (def)
>>> +                    || !flow_bb_inside_loop_p (loop, gimple_bb (def)))
>>> +                  continue;
>>> +              }
>>> +              hoist = false;
>>> +              break;
>>> +            }
>>> +
>>> +            if (hoist)
>>> +            {
>>> +              basic_block preheader = loop_preheader_edge (loop)->src;
>>> +              gimple_stmt_iterator si_dst = gsi_last_bb (preheader);
>>> +              gsi_move_after (&si, &si_dst);
>>> +              continue;
>>> +            }
>>> +          }
>>> +          gsi_next (&si);
>>> + }
>>> +    }
>>> +
>>>    /* End loop-exit-fixes after versioning.  */
>>>
>>>    if (cond_expr_stmt_list)
>>> Index: gcc/ChangeLog
>>> ===================================================================
>>> --- gcc/ChangeLog (revision 202663)
>>> +++ gcc/ChangeLog (working copy)
>>> @@ -1,3 +1,8 @@
>>> +2013-10-01  Cong Hou  <congh@google.com>
>>> +
>>> + * tree-vect-loop-manip.c (vect_create_cond_for_alias_checks): Combine
>>> + alias checks if it is possible to amortize the runtime overhead.
>>> +
>>>
>>>
>>
>> --
>> Richard Biener <rguenther@suse.de>
>> SUSE / SUSE Labs
>> SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
>> GF: Jeff Hawn, Jennifer Guild, Felix Imend